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Friday, June 16, 2017

Signs of past California 'mega-quakes' show danger of the Big One on San Andreas fault

As Interstate 10 snakes through the mountains and
toward the golf courses, housing tracts and resorts of the Coachella
Valley, it crosses the dusty slopes of the San Gorgonio Pass.

The
pass is best known for the spinning wind turbines that line it. But for
geologists, the narrow desert canyon is something of a canary in the
coal mine for what they expect will be a major earthquake coming from
the San Andreas fault.
The pass sits at a key geological point,
separating the low desert from the Inland Empire, and, beyond that, the
Los Angeles Basin.
Through it runs an essential aqueduct that
feeds Southern California water from the Colorado River as well as vital
transportation links. It’s also the path for crucial power transmission
lines.
California earthquake experts believe what happens at the
San Gorgonio Pass during a major rupture of the San Andreas fault could
have wide-ranging implications for the region and beyond.
They worry a huge quake could sever lifelines at the pass forweeks
or months, cutting Southern California off from major highway and rail
routes as well as sources of power, oil and gas. Southern California’s
cities are surrounded by mountains, making access through narrow passes
like the San Gorgonio essential.

Experts have also expressed grave concerns about the Cajon Pass,
where Interstate 15 and key electric and fuel lines run. Other problem
spots are the Tejon Pass, through which Interstate 5 passes, and the
Palmdale area, through which the California Aqueduct crosses.
One
of the most dire scenarios geologists have studied is a quake that
begins at the Salton Sea. Such a quake would be particularly dangerous
because the fault’s shape points shaking energy toward Los Angeles.
Southern
California has not seen an earthquake like this since humans started
recording history here. But the geological evidence of such quakes is
all around us.

Where electric lines cross the San Andreas fault

Where electrical transmission lines cross the San Andreas fault, according to a 2008 report. (U.S. Geological Survey / ShakeOut)

Signs of megaquakes

In
Desert Hot Springs, hints of the mighty San Andreas fault lie all over:
The rise of mountains that created the Coachella Valley. The oases and
palm trees — made possible only because earthquakes pulverized rocks
that allowed springs to burst to the surface.
A geologist’s
trained eye can even spot exactly where the fault is located. In one
exposed cliff, USGS research geologist Kate Scharer showed how one side
of a hill has moved northward and skyward compared with the right side —
and the gouge in the hillside between them was the fault.
Seismologist Lucy Jones stands on top of the San Andreas fault, which
has pushed up the left side of the hill northward and higher than the
side to the right from past earthquakes. (Photo by Allen J. Schaben /
Annotation by Raoul Ranoa / Los Angeles Times)

Farther away, Scharer described how an old lower canyon was severed from the upper canyon and its ancient source of water.
The old path of the Pushawalla Canyon was carved into Earth 32,000 years
ago, according to U.S. Geological Survey research geologist Kate
Scharer. Since then, the lower part of the canyon has been moving left,
to the northwest, and has moved half a mile so far. (Kate Scharer / U.S.
Geological Survey)

Direction matters

There’s a reason why this particular scenario vexes scientists:
An
earthquake arriving from this direction would point cataclysmic shaking
directly into the heart of L.A., a kind of disaster that has not been
seen since humans began recording history in California. Shaking could
last for as long as three minutes.
In a magnitude 8.2 scenario,
the earthquake would begin at the Salton Sea, and then — like a big rig
driving on a freeway — speed up the San Andreas fault toward Los Angeles
County.
“It’s shooting all of that energy straight into the L.A. Basin,” Scharer said.

Why a quake that begins so far away matters

An earthquake that begins more than 100 miles from L.A. might seem like something you might not worry about.
But a magnitude 8.2 earthquake is no ordinary earthquake.
The traditional image of an earthquake might be to show the epicenter — the point at which the earthquake begins. USGS, Mapzen, OpenStreetMap, Angelica Quintero / Los Angeles Times)

But that doesn’t tell the whole story.
A better representation
of a large earthquake would show how the earthquake travels up the
fault. And this becomes more important for large earthquakes, which
require an incredible amount of area in which the sides of the fault
move against each other.
So, according to seismologist Lucy Jones, if a San Andreas earthquake began at the Salton Sea and ...
♦ ended at Mount San Gorgonio, it would be a 7.3 earthquake. (U.S. Geological Survey, Mapzen, OpenStreetMap, Angelica Quintero / Los Angeles Times)
♦ stopped at the Cajon Pass, it would be a magnitude 7.6 or 7.7 seismic event.

♦ and “if it goes all the way from the way from the Salton Sea to near
Paso Robles, we’d get an 8.2. So that’s probably the biggest we can
have,” Jones said.

“I think it’s going to go all the way to Paso Robles,” Jones said of the next Big One.
Jones cited a recent study
by Scharer that found that earthquakes happen at the San Andreas around
the Grapevine on average every 100 years. It has been 160 years since
the last major earthquake on that section of the fault.

Hope for L.A.

Here
in the Coachella Valley and across the West Coast, scientists have been
busy installing new seismic equipment as they construct an earthquake
early warning system, which could give places like L.A. seconds — or
even a minute or more — of warning before the shaking waves arrive from
an earthquake.
The project, however, is in danger of losing
funding. President Trump’s proposed budget suggests ending federal
funding for the early warning system. Southern California’s elected
officials in Congress have voiced support for continuing funding of the
project.Aris
Aspiotes, a field engineer with the USGS, shows off an earthquake
sensor that measures the movement of ground on the San Andreas fault.
(Allen J. Schaben / Los Angeles Times)Here are
some more answers to questions given by Jones and Scharer as they gave a
tour to elected officials on a trip organized by the Southern
California Assn. of Governments:

Why are we so concerned about the San Andreas fault, when other faults are closer to cities?

The
worst thing about an 8.2 on the San Andreas is that all of Southern
California would be hit hard at the same time. San Bernardino, for
instance, wouldn’t be able to call for help from Los Angeles, which
would have its own problems.
“With 300 miles of fault all going in
the same earthquake, you then have everybody affected at the same
time,” Jones said. “The San Andreas is the one that will produce the
earthquake that’s going to cause damage in every city” in Southern
California — including Santa Barbara and San Diego.

Why is the San Andreas considered so likely to rupture?

Because it’s California’s fastest-moving fault.
“It’s a little bit like — the moron who is driving the fastest is the most likely to get into an accident,” Scharer said.

If a couple were holding hands across the San Andreas fault, what would happen when the earthquake hits?

Here
in Desert Hot Springs, the couple would be thrown down. The ground
would shatter. And in a matter of seconds the two would be separated by
as much as 30 feet, Scharer said, almost the entire length of a city
bus.
One would lurch toward San Francisco, and the other toward the Mexican border.

Can the San Andreas trigger aftershocks on other faults closer to the city?

Yes.
One scenario of a San Andreas earthquake results in aftershocks on the
Newport-Inglewood fault, which runs between L.A.’s Westside through
Orange County, and the Sierra Madre fault in the San Gabriel Valley. “We
even had one in Sacramento,” Jones said.
Even the Hayward fault
in the San Francisco Bay Area could be set off by an earthquake on the
southern San Andreas fault, Jones said.
This has happened before.
The great 1906 San Francisco earthquake, estimated at being magnitude
7.7 to 7.9, sent a 5.5 aftershock to Santa Monica Bay and a magnitude 6
earthquake to Imperial County, near the Mexican border.

Can you explain how the San Andreas fault works?

Western
California — San Diego, Los Angeles, Santa Barbara — is moving to the
northwest. Areas to the east of the fault are moving to the southeast.Los
Angeles is slowly moving closer to San Francisco as a result of
earthquake activity on the San Andreas fault. (U.S. Geological Survey)

How fast has the San Andreas fault moved in the last million years?

It has moved about 22 miles in the last million years, Jones said.

When will the Big One hit?

The San Andreas fault does not slice under the city of Los Angeles. So why should Angelenos worry?

Los Angeles sits on a basin filled with sand and gravel.
So
when shaking waves come, they “bang up against the side of the
mountains and reverberate back out across the basin,” Scharer said.
“Those waves are very effective at traveling through piles of gravel.”

Can scientists develop something that could absorb all the shaking energy from a massive earthquake before the city is hit?

No. The energy produced by a large San Andreas earthquake, “it’s like the size of a small nuclear bomb,” Scharer said.
An 8.2 earthquake would produce far more energy than what was produced by the nuclear bomb dropped on Hiroshima.

Do small earthquakes relieve pressure on the faults?

No.
“Little earthquakes don’t get rid of big ones,” Jones said. “The more
little earthquakes you have, the more you have to have bigger ones.”

How should cities cope with the earthquake risk?

Jones
said utilities, such as water, electricity and gas, require more
attention. “If we don’t deal with utilities … we aren’t going to be able
and stay here and work,” she said.

Are California’s building codes equipped to deal with big earthquakes?

A
few California cities have boosted safety regulations for older
buildings in response to earthquakes. In recent years, several cities,
including Los Angeles and San Francisco, began requiring retrofits of
vulnerable apartment buildings. L.A. is even requiring retrofits of
brittle concrete buildings.
But Jones is critical of minimum
building standards for new construction in California, which she said
allow for a 10% chance of new buildings collapsing and killing people in
an earthquake.
Jones favors increasing standards for new
construction, ordering new buildings designed so that they can be
immediately occupied after an earthquake. She said that would increase
costs by 1%.
“I think you need to be safe enough to walk into a
building, so that you don’t lose the use of it — and so your neighbors
don’t lose the use of their buildings,” she said.

Are new buildings built better elsewhere?

Jones
says new buildings are stronger, for example, in Chile. That’s because
the country makes those who build new buildings responsible if the
structure suffers earthquake damage in the first decade after it is
completed.
As a result, owners have insisted on strong
construction, Jones said. And the country rode out a recent magnitude
8.8 earthquake well.